1. Field of the Invention
This invention relates to Low Noise Amplifiers (LNAs) and has particular relation to LNAs which are highly linear.
2. Background Art
In wireless applications it is important to constrain power consumption. At the same time, the operating environment often dictates very high performance for the RF front-end. This is particularly accentuated in the case of CDMA systems, because they operate full-duplex (i.e. receiving and transmitting at the same time). It can be shown that, simultaneously with a low noise figure, the LNA must also have very high IIP3. See, for example, V. Aparin, B. Butler, Paul Draxler, xe2x80x9cCross Modulation Distortion in CDMA Receiversxe2x80x9d, IEEE International Microwave Symposium, Boston, June 2000. In a bipolar transistor design, the IIP3 typically increases with increasing current consumption but so does the noise figure at high currents. Thus, in a standard design, high IIP3 often results in poor noise figure and excessive current consumption.
There are several conventional techniques to obtain higher IIP3 without sacrificing noise figure or current consumption.
The IIP3 at RF frequencies is strongly affected by the presence of low-frequency distortion products. See, for example, V. Aparin, C. Persico, xe2x80x9cEffect of Out-of-Band Termination on Intermodulation Distortion in Common-Emitter Circuitsxe2x80x9d, IEEE MTT-S Dig, vol. 3, June 1999, pp. 977-980. In a two-tone test, nonlinearities will cause the generation of several mixing products. One of these occurs at (f1xe2x88x92f2) where f1 and f2 are the frequencies of the two tones. This can be viewed as a low-frequency modulation of the operating point. Due to unavoidable internal feedback inside the transistor as well as external feedback, the (f1xe2x88x92f2) product will mix again with f1 and f2, thus creating in-band distortion products at the IM3 frequencies (f1xe2x88x92f2)=f1=2*f1xe2x88x92f2 and (f2xe2x88x92f1)+f2=2*f2xe2x88x92f1. In order to obtain high IIP3, it is therefore beneficial to ensure that a low impedance is presented to these products, essentially shorting them out.
It is a common design approach of power amplifiers to reduce the low-frequency impedance of the bias circuits as much as possible to avoid the bias modulation. See, for example, F. N. Sechi, xe2x80x9cLinearized Class-B Transistor Amplifiersxe2x80x9d, IEEE J. Solid-State Circuits, vol. 11, April 1976, pp. 264-270.
The low-frequency impedance approach has not been previously used in the design of LNAs (as distinct from bias circuits). As it will be seen later the improvement can be quite dramatic.
In addition to this technique, we also take advantage of the fact that the chosen two-stage architecture allows tailoring of the inter-stage matching network to provide optimum IIP3. This is utilized in a technique explained below in xe2x80x9cGraphical Design Techniquexe2x80x9d.